linux-kernel - Re: [RFC PATCH] LTTng relay buffer allocation, read, write

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Message-ID: <20080930183531.GA20670@Krystal>
Date:	Tue, 30 Sep 2008 14:35:31 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Martin Bligh <mbligh@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, prasad@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>, od@...e.com,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, hch@....de,
	David Wilder <dwilder@...ibm.com>,
	Tom Zanussi <zanussi@...cast.net>
Subject: Re: [RFC PATCH] LTTng relay buffer allocation, read, write

* Steven Rostedt (rostedt@...dmis.org) wrote:
> 
> On Tue, 30 Sep 2008, Mathieu Desnoyers wrote:
> > 
> > In Steven's scheme, the event IDs in the 4 bytes are reserved for
> > (useless) internal use ;) They can therefore not be used for specific
> > tracer event IDs, which I think is a misuse of the precious bits
> > otherwise available to store really useful event IDs.
> 
> I'm using them, so they must not be totally useless. ;-)
> 
> But ftrace has its own event ids and I don't want the ring buffer to ever 
> have to know about them.
> 
> -- Steve
> 

You are actually using them to put redundant information that could be
encoded differently and thus save 4 bits per event records, more or less
what will be needed by most tracers (15 IDs, 1 reserved for an extended
ID field).

So the fact that you use them does not mean they are really required,
and I don't think such duplicated information actually makes things more
solid. Maybe just more obscure ?

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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